1. Field of Invention
The present invention generally relates to an organic electroluminescence device, and more particularly relates to an organic blue luminescence material applicable to an electroluminescence device. The material is highly efficient, lower-voltage operative and easy to be synthesized.
2. Related Art
The first efficient green organic electroluminescence (EL) device developed by Eastman Kodak Company in 1987 started to arouse people""s attentions. The basic construction of an organic electroluminescence device is shown in FIG. 1. There are thin organic films or called electroluminescence element arranged between a cathode 3 and a anode 1. Electric current passing through the EL element makes electrons combining with electric holes at the EL element and emitting light.
The two-layer structure of an organic electroluminescence device of FIG. 1 has a two-layer organic electroluminescence element. A hole-transport layer 6 is deposited on a transparent anode 1 (which is an indium-tin-oxide (ITO) conductive film formed on a glass substrate 2). A light emitting layer 5 and an electron-transport layer are further deposited on the hole-transport layer 6. The light emitting layer 5 and the electron-transport layer 4 are capable of transporting electrons and emitting. A metal cathode 3 is finally deposited on the electron-transport layer 4. The cathode 3 mainly includes metallic elements of magnesium, lithium and aluminum. A multi-layer structure is shown in FIG. 2. There are other organic layers deposited between the transparent anode 1 and the metal cathode 3. They are hole-injection layer HIL 7, hole-transport layer HTL 6, light emitting layer EML 5 and electron-transport layer ETL (or electron-injection layer EIL) 4, etc. These layers will increase the injection efficiency of carriers (electrons and holes), decrease the operating voltage and increase the carrier recombination probability.
EL devices have advantages of high luminance, self-emission, low driving voltage, no limitation of view angle and easy fabrication. Therefore, they are applicable to planar displays. But, there are still some difficulties, for example, lower efficiency of emission, limited luminance, and limited durability. Influencing factors for these problems mainly include:
1) The efficiency of carrier injection. Since the EL device is a light emitter having two carrier injections that electrons and holes are injected from the cathode and the anode to the organic layers where recombination occurs, energy released and light emitted, the capability or efficiency of the electrode injections will highly influence the luminance and efficiency of the light emission. Therefore, some new materials for injection layers, such as the material for hole injection layer, disclosed in Taiwan Patent Application No. 8610155 1, are developed for increasing the efficiency of injection.
2) The impurities in the EL elements. When the carriers recombining in the EL element and emitting the energy, the impurities will transform a part of energy into heat, and decrease the efficiency and luminance of light emission. The heat accumulation also causes recrystallization of the organic element, and even causes heat decomposition and influence the durability of the element.
In the recent display using organic light emitting diodes (OLED), single color, multiple-color or full color types are developed. The construction of a full color OLED display can be one of the following three types, as shown in FIGS. 3A to 3C:
1) Three color independent emitters. For each pixel, three EL members OEL1, OEL2 and OEL3 for generating red, green and blue lights are arranged so as to compose the desired color of light.
2) Blue light transformers. Providing a blue EL member (B_OEL) for a blue light source.
Preparing light transforming films F1 for transforming the blue light into red and green lights. The blue EL member here is a full area member without the need of making pixels.
3) White light color filters. A white EL member (W-OEL) provides white back-light for LCD color filters F2 to provide full colors.
The former two types have been more quickly developed and products are marketed. When using blue light transformers, a high efficiency blue EL is needed, But generally, the common blue EL device has lower efficiency and requires higher operating voltage. The composition of blue organic EL element is also complicated, with lower yield and higher cost. Known blue EL elements include 9, 10-di-(2-naphthyl)anthracene derivatives as disclosed in U.S. Pat. No. 5,935,721, and 9, 10-bis(3xe2x80x2, 5xe2x80x2-diaryl)phenylanthracene derivatives as disclosed in U.S. Pat. No. 5,972,247. But, the two prior blue EL elements are not quite satisfied.
It is therefore an object of the present invention to provide a blue EL element with high efficiency of emission, low operatng voltage, and easy to be synthesized.
A blue EL element with high efficiency of emission and low driving voltage according to the present invention is an adjusted construction of 9, 10-(diphenyl)anthracene). An asymmetric construction is made, and a steric-hindrance substitute group is added in order to prevent the element from recrystallization.
The EL element according to the present invention is made of a substance represented by the following chemical formula: 
in which m, n=1-3; the substitute R is selected from the group consisting of t-butyl, phenyl, methylphenyl, naphehyl and heterocyclic compound.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.